Diatomaceous earth, also known as D.E., or diatomite, is a naturally occurring sedimentary rock which may be crumbled into a fine powder. The composition of diatomaceous earth is mainly amorphous silica formed from remains of diatoms, algae with siliceous skeletons. The unique porous silica structure of diatomaceous earth may allow for high absorptive capacity and surface area, chemical stability, and low bulk density. The properties may enable natural and/or processed diatomaceous earth to find applications as filtration media, absorbents for liquids, porous supports for catalysts, carriers for pesticides, fillers in paints and paper, and refractory or abrasive products in a variety of industries.
The natural diatomaceous earth has low permeability (i.e. between about 0.01 darcy and about 0.10 darcy) due to the particle size distribution and the inherent porous structure of the diatoms. To increase the permeability, the natural diatomaceous earth has to be further processed, for instance, by heating, to sinter the diatoms, dehydrate and reduce the specific surface area of the particles.
In practice, when preparing diatomaceous earth filter aids with high permeability, the diatomaceous earth from ore is transported to a mill, crushed, ground, screened, pre-treated to remove extraneous material or unwanted ingredients, and then calcined at temperatures greater than about 1000° C. (1832° F.) in a rotary kiln or calciner. If the crude material is only calcined without adding a flux agent, the ensuing products are called calcined diatomaceous earth. If a fluxing agent is added to the rotary calciner during the calcination process, the resulting products are called flux-calcined diatomaceous earth. The purpose of the calcination step is to create a better filtering agent with desired permeabilities and agglomeration forms by further hardening the exoskeletons of the diatoms and forming coarse agglomerates of individual diatoms. Calcination may also oxidize organic substances and convert or decompose various undesirable inorganic compounds into oxides, silicate, or aluminosilicates.
Since diatom skeletons comprise mainly amorphous silica, the naturally-occurring diatomaceous earth is largely amorphous with a small amount of crystalline silica in the forms of quarts or cristobalite. The amount of crystalline silica (about 0.0-about 1.1 wt %) in natural diatomaceous earth is relatively small. However, more crystalline silica, mainly cristobalite, can be formed when the diatomaceous earth is calcined at temperatures above 1000° C. (1832° F.). It is known that the crystallization of silica (cristobalite) from amorphous silica is extremely slow below 600° C. (1112° F.), remains slow up to 850° C. (1562° F.), but increases very rapidly at temperatures above 900° C. (1652° F.). Consequently, conventional calcination methods may produce diatomaceous earth filter aids containing between about 20 wt % and about 75 wt % of crystalline silica. Such high amount of crystalline silica in calcined diatomaceous earth may be undesirable in some applications.
Currently one type of flux calcination process uses sodium carbonate or other sodium compounds as fluxing agents to fuse diatomaceous earth and increase the permeability, thereby producing fast grades of filter aids. However, the use of sodium compounds during calcination results in an undesirable increase in the amount of crystalline silica, mainly cristobalite, in the calcined final product.
To better answer the challenges raised by the filter aid industry to produce fast, permeable diatomaceous earth products, there is a need to develop a manufacturing method which leads to the production of calcined diatomaceous earth products with high permeability (for example, greater than about 1.0 darcy) and low crystalline silica content (for example, lower than about 4 wt %).